The present invention was made in connection with a grant from the National Institute of Health Grant R21 CA88231
Permanent interstitial implantation of iodine or palladium radioactive seeds is currently used for the treatment of prostate cancer. In connection with the implantation of such brachytherapy seeds a transrectal ultrasound (TRUS) examination of the prostate is used in the operating room. The TRUS probe is mounted on a fixture that contains a needle guidance template, which rests against the perineum and incorporates detents enabling the probe to be introduced into or withdrawn from the rectum in 5-mm steps. Scans are oriented in a transverse plane perpendicular to the probe axis. The fixture is advanced into the rectum until the seminal vesicles are imaged and scans are performed in planes separated by 5 mm as the probe is withdrawn. During the scans, the prostate is constrained against movement by position-fixing needles.
TRUS images are ported through a standard output jack to a laptop computer on which treatment-planning software is run. Such software may, for example, be VariSeed provided by Varian Medical Systems, Inc. (VMSI), Charlottesville, Va. This treatment planning software generates an image of available and optimal needle locations, which are fixed by the template used for needle guidance. The oncologist demarcates the prostate in each scan plane and prescribes a radiation dose for the glad as a whole. The planning software then presents a set of suggested seed positions in each scan plane, which the oncologist can accept or reject based on isodose distributions plotted by the software for whatever seed positions are chosen.
The software bases the isodose distributions on seed locations in the 3-D volume spanned by the set of scan planes in the planning set. Immediately after planning is completed, seed-implantation needles are loaded with the radioactive seeds and plastic spacer seeds in a manner that places radioactivity at the specified depths, corresponding to the scan planes, along each needle position. The spacers are not visible in ultrasonic or x-ray computed tomography (CT) images. Loaded needles are then inserted into the prostate via the perineum through the template holes that match the needle positions depicted on the planning-software image. Dosimetric evaluation subsequently is performed using post-implant CT imaging. Traditionally, post-implant CT scans are performed within two weeks of implantation, but in some cases they are performed within 24 hours of implantation.
In some cases, the CT scans show that the actual location of implanted seeds differs from their planned locations. Studies by Potters, et al., have shown that 30% of prostate brachytherapy procedures result in a dose to 90% of the prostate that is less than the prescribed dose. [Potters, et al., Int. J. Radiat. Oncol. Biol. Phys., 50:605–614, 2001] Studies by Stock, et al., showed that 32% of under-dosed patients have biochemical failure (as evidenced by a rise in the blood level of prostate-specific antigen (PSA)) within four years, where as only 8% of properly dosed patients have biochemical failure. [Stock, et al, Int. J. Radiat. Oncol. Biol. Phys., 41:101–108, 1998] The conventional B-mode ultrasound images generated at the time of the procedure do not allow adequate visualization of the placed seeds because of clutter, shadowing and the loss of echo signals due to seed angulation. Clutter often increases immediately during the procedure from hemorrhage and edema caused by the trauma of needle insertion.
It is an object of the present invention to provide improved imaging of implanted seeds during the implantation process with the result that seed implantation errors can be corrected, for example by implantation of additional seeds during the procedure.